This invention relates to the indirect electrosynthesis of carbonyl containing compounds via oxidation of aromatic and alkyl substituted aromatic compounds using ceric ions. More specifically, the invention relates to a faster and more electrically efficient indirect electrochemical process for the production of carbonyl containing compounds such as naphthoquinone from starting materials such as naphthalene, in conjunction with the electrochemical regeneration of the oxidant ion.
An indirect oxidation of organic compounds using ceric ions in aqueous methanesulfonic acid is described in U.S. Pat. Nos. 4,639,298, 4,670,108 and 4,701,245 to Kreh and others. A paper which describes further work in this area is Mediated Electrosynthesis With Cerium (IV) In Methanesulfonic Acid by Spotnitz, et. al., J. Appl. Electrochem 90 (2) 1990, p. 209. These documents detail the advantages of using aqueous solutions of cerium methanesulfonate and methanesulfonic acid to oxidize aromatic and alkyl substituted aromatic compounds to carbonyl containing compounds by contacting the reactant compounds with the ceric methanesulfonate solution. The present invention relates to improvements in the speed and efficiency of the processes described in these documents.
Prior to the work described in the noted prior art, the use of ceric ions for oxidation of such organic compounds was inefficient because of the inability to create relatively concentrated oxidizing solutions which could be electrochemically regenerated. Ceric and cerous sulfate have limited solubility, making the electrochemical regeneration process inefficient.
While the reaction of ceric solutions with polycyclic aromatic hydrocarbons has been known for some time and workers have operated the process from laboratory to pilot plant, no one has taken the process to commercial success. The capital and operating costs of the process have been too unattractive for the investment to be made. These problems stem from the plant size required to operate the process with very dilute solutions, less than 1 molar in ceric ion. Current efficiencies are low and the plant vessels are necessarily large to accommodate such a high volume of electrolyte.
The use of methanesulfonic acid described by Kreh et. al. addressed this problem to some extent, since operating concentrations were raised from 0.1 molar to 5 molar in cerium. This process improved the convenience of the procedure as the working solution was capable of oxidizing much more organic substrate before the regeneration step was required. The penalty incurred was the cost of a more expensive electrolyte and complications in the separation procedure. Additionally, the process of the prior art is not as energy efficient as would be desired. Thus, while the oxidation reactions described in the prior art brought about substantial improvements, there continues to be a need for an oxidizing process which is sufficiently fast and electrochemically efficient enough for commercialization.